A threaded portion of a tap used for cutting an internal thread in a workpiece consists of a lead part formed on the leading side and a complete thread part formed on the following side.
When such a tap is used to cut an internal thread in a workpiece, its lead part first cuts into the workpiece thereby gradually generating a thread groove, and the complete thread part then makes thread engagement with the groove to act as a guide guiding the advancing movement of the tap itself.
However, the tap used for cutting the internal thread in a workpiece has various different angular factors, such as, the rake or hook angle of each cutting edge at its lead part, the radial angle on the lead as well as the inclination angle at the circumference of each cutting edge, and the radial thread relief angle at the flank face of each cutting edge relative to the normal helical direction of the thread. In addition, the thrust that may be large or small is imparted to the tap itself. As a result, the advancing movement of the tap in its axial direction tends to fluctuate, and the force of reaction due to the resistance to cutting tends to act on the tap. This leads to such a problem that the front end of the generated internal thread engaged by the lead part of the tap tends to be widened or flared, resulting in an undesirable degradation of the dimensional precision of the flank faces of the generated internal thread.
With a view to deal with such a problem, various proposals for improvements have been made, as disclosed in, for example, JP-A-60-194425 (UM) which relates to a spiral fluted tap. According to the disclosure of the cited application, a planar negative relieving having a substantially constant width is formed to extend from the root towards the crest on the leading flank or the following flank along the line of intersection between the wall face of the flute and the flank of each thread ridge. However, it is apparent that, according to those proposals intending improvements, a large resistance is encountered during the cutting operation, because the negative relieving described above acts to effect plastic cutting rather than mechanical cutting. As a result, the phenomenon of frictional fusion similar to that encountered with a built-in edge of a turning tool tends to occur at the relieved portion. Therefore, with the increase in the number of times of use (cutting) of the tap, the dimensional precision of the flank faces of the finished internal thread has been inevitably degraded together with the shortened useful service life of the tap itself.